U.S. patent number 8,102,943 [Application Number 11/740,036] was granted by the patent office on 2012-01-24 for variable digital very low intermediate frequency receiver system.
This patent grant is currently assigned to RF Micro Devices, Inc.. Invention is credited to Alexander Wayne Hietala, Nadim Khlat.
United States Patent |
8,102,943 |
Khlat , et al. |
January 24, 2012 |
Variable digital very low intermediate frequency receiver
system
Abstract
The present invention is a radio frequency (RF) receiver that
uses an RF mixer for tuning to desired frequency bands. The RF
receiver down converts a received RF signal into a very low
intermediate frequency (VLIF) signal. When receiving a desired RF
signal, the frequency of the resulting VLIF signal is called the
desired VLIF frequency, and is based on the signal strength of the
received RF signal. In one embodiment of the present invention, the
desired VLIF frequency is selected to be one of two VLIF
frequencies, and is inversely related to the signal strength of the
received RF signal. For example, a higher desired VLIF frequency is
selected when receiving lower signal strength RF signals to
increase effective receiver sensitivity. A lower desired VLIF
frequency is selected when receiving higher signal strength RF
signals to improve image rejection.
Inventors: |
Khlat; Nadim (Midi-Pyrenees,
FR), Hietala; Alexander Wayne (Phoenix, AZ) |
Assignee: |
RF Micro Devices, Inc.
(Greensboro, NC)
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Family
ID: |
45476850 |
Appl.
No.: |
11/740,036 |
Filed: |
April 25, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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60798577 |
May 8, 2006 |
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Current U.S.
Class: |
375/316;
375/344 |
Current CPC
Class: |
H04B
1/0007 (20130101) |
Current International
Class: |
H04L
27/00 (20060101); H03K 9/00 (20060101) |
Field of
Search: |
;375/316 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Payne; David C.
Assistant Examiner: File; Erin
Attorney, Agent or Firm: Withrow & Terranova,
P.L.L.C.
Parent Case Text
RELATED APPLICATIONS
This application claims the benefit of provisional patent
application Ser. No. 60/798,577, filed May 8, 2006, the disclosure
of which is hereby incorporated by reference in its entirety.
Claims
What is claimed is:
1. A radio frequency (RF) receiver comprising: RF mixer circuitry
adapted to: receive an RF input signal; receive a frequency select
signal; and provide a down converted output signal based on mixing
the RF input signal with a local oscillator signal having a local
oscillator frequency, which is a function of the frequency select
signal; and control circuitry adapted to: receive an RF signal
strength signal based on a signal strength of the RF input signal,
wherein the RF signal strength signal is different from the RF
input signal; and generate the frequency select signal based on the
RF signal strength signal to cause the RF mixer circuitry to down
convert the RF input signal to a very low intermediate frequency
(VLIF) signal.
2. The RF receiver of claim 1 wherein the RF mixer circuitry
further comprises: a single in-phase mixer adapted to provide an
in-phase down converted output signal; and a single
quadrature-phase mixer adapted to provide a quadrature-phase down
converted output signal.
3. The RF receiver of claim 2 wherein: the single in-phase mixer is
further adapted to receive an in-phase local oscillator signal; and
the single quadrature-phase mixer is further adapted to receive a
quadrature-phase local oscillator signal, wherein the in-phase
local oscillator signal and the quadrature-phase local oscillator
signal are approximately equal in amplitude and phase-shifted
approximately 90 degrees from each other.
4. The RF receiver of claim 2 further comprising a quadrature
filter circuit adapted to: receive the in-phase down converted
output signal; filter the in-phase down converted output signal to
remove unwanted signals to create a filtered in-phase down
converted output signal; receive the quadrature-phase down
converted output signal; and filter the quadrature-phase down
converted output signal to remove unwanted signals to create a
filtered quadrature-phase down converted output signal.
5. The RF receiver of claim 2 further comprising a quadrature gain
correction circuit adapted to: receive the in-phase down converted
output signal; receive the quadrature-phase down converted output
signal; apply an amplitude adjustment to the in-phase down
converted output signal to create a corrected in-phase down
converted output signal; and apply an amplitude adjustment to the
quadrature-phase down converted output signal to create a corrected
quadrature-phase down converted output signal, wherein the
corrected quadrature-phase down converted output signal and the
corrected in-phase down converted output signal are approximately
equal in amplitude.
6. The RF receiver of claim 2 further comprising a quadrature gain
correction circuit adapted to apply an amplitude adjustment such
that the quadrature-phase down converted output signal and the
in-phase down converted output signal are approximately equal in
amplitude.
7. The RF receiver of claim 2 further comprising a quadrature phase
correction circuit adapted to: receive the in-phase down converted
output signal; receive the quadrature-phase down converted output
signal; apply a phase adjustment to the in-phase down converted
output signal to create a corrected in-phase down converted output
signal; and apply a phase adjustment to the quadrature-phase down
converted output signal to create a corrected quadrature-phase down
converted output signal, wherein the corrected quadrature-phase
down converted output signal and the corrected in-phase down
converted output signal are phase-shifted approximately 90 degrees
from each other.
8. The RF receiver of claim 2 further comprising a quadrature phase
correction circuit adapted to apply a phase adjustment such that
the quadrature-phase down converted output signal and the in-phase
down converted output signal are phase-shifted approximately 90
degrees from each other.
9. The RF receiver of claim 2 further comprising a quadrature
analog-to-digital conversion circuit adapted to: receive the
in-phase down converted output signal; convert the in-phase down
converted output signal from an analog signal into a digital signal
to create a digital in-phase down converted output signal; receive
the quadrature-phase down converted output signal; and convert the
quadrature-phase down converted output signal from an analog signal
into a digital signal to create a digital quadrature-phase down
converted output signal.
10. The RF receiver of claim 2 further comprising: a quadrature
filter circuit adapted to: receive the in-phase down converted
output signal; filter the in-phase down converted output signal to
remove unwanted signals to create a filtered in-phase down
converted output signal; receive the quadrature-phase down
converted output signal; and filter the quadrature-phase down
converted output signal to remove unwanted signals to create a
filtered quadrature-phase down converted output signal; and a
quadrature analog-to-digital conversion circuit adapted to: receive
the filtered in-phase down converted output signal; convert the
filtered in-phase down converted output signal from an analog
signal into a digital signal to create a digital in-phase down
converted output signal; receive the filtered quadrature-phase down
converted output signal; and convert the filtered quadrature-phase
down converted output signal from an analog signal into a digital
signal to create a digital quadrature-phase down converted output
signal.
11. The RF receiver of claim 1 wherein the down converted output
signal further comprises a desired VLIF frequency, wherein the
desired VLIF frequency is inversely related to a magnitude of the
RF signal strength signal.
12. The RF receiver of claim 1 wherein the down converted output
signal further comprises a desired VLIF frequency, wherein the
desired VLIF frequency is one of a plurality of frequencies.
13. The RF receiver of claim 1 wherein the down converted output
signal further comprises a desired VLIF frequency, wherein the
desired VLIF frequency is a first VLIF frequency if the RF signal
strength signal is greater than or equal to a threshold signal
strength, and the desired VLIF frequency is a second VLIF frequency
if the RF signal strength signal is less than the threshold signal
strength.
14. The RF receiver of claim 13 wherein the first VLIF frequency is
approximately 120 kilohertz, the second VLIF frequency is
approximately 175 kilohertz, and the threshold signal strength is
approximately -93 decibel milliwatt (dbm).
15. A method comprising: receiving a radio frequency (RF) input
signal; receiving a frequency select signal; providing a down
converted output signal based on mixing the RF input signal with a
local oscillator signal having a local oscillator frequency, which
is a function of the frequency select signal; receiving an RF
signal strength signal based on a signal strength of the RF input
signal, wherein the RF signal strength signal is different from the
RF input signal; and generating the frequency select signal based
on the RF signal strength signal to cause down conversion of the RF
input signal to a very low intermediate frequency (VLIF)
signal.
16. The method of claim 15 wherein the down converted output signal
further comprises an in-phase down converted output signal and a
quadrature-phase down converted output signal.
17. The method of claim 16 further comprising: receiving the
in-phase down converted output signal; filtering the in-phase down
converted output signal to remove unwanted signals to create a
filtered in-phase down converted output signal; receiving the
quadrature-phase down converted output signal; and filtering the
quadrature-phase down converted output signal to remove unwanted
signals to create a filtered quadrature-phase down converted output
signal.
18. The method of claim 16 further comprising: receiving the
in-phase down converted output signal; receiving the
quadrature-phase down converted output signal; applying an
amplitude adjustment and a phase adjustment to the in-phase down
converted output signal to create a corrected in-phase down
converted output signal; and applying an amplitude adjustment and a
phase adjustment to the quadrature-phase down converted output
signal to create a corrected quadrature-phase down converted output
signal, wherein the corrected quadrature-phase down converted
output signal and the corrected in-phase down converted output
signal are approximately equal in amplitude and phase-shifted
approximately 90 degrees from each other.
19. The method of claim 16 further comprising: receiving the
in-phase down converted output signal; converting the in-phase down
converted output signal from an analog signal into a digital signal
to create a digital in-phase down converted output signal;
receiving the quadrature-phase down converted output signal; and
converting the quadrature-phase down converted output signal from
an analog signal into a digital signal to create a digital
quadrature-phase down converted output signal.
20. The method of claim 15 wherein the down converted output signal
further comprises a desired VLIF frequency, wherein the desired
VLIF frequency is inversely related to a magnitude of the RF signal
strength signal.
Description
FIELD OF THE INVENTION
The present invention relates to radio frequency (RF) receivers
used in RF communications systems.
BACKGROUND OF THE INVENTION
Many RF communications systems have RF receivers that need to
receive a desired RF signal on a specific RF channel, which is a
desired RF channel that has a desired bandwidth and a desired RF
center frequency. One function of the RF receiver is to reject any
RF signals at frequencies other than those within the desired
bandwidth of the desired RF channel; therefore, numerous filtering
and signal rejection techniques have been developed to achieve this
function. One such technique is called super-heterodyning, in which
received RF signals are filtered and then mixed with a local
oscillator signal to down convert the filtered RF signals into
lower frequency signals, which are known as intermediate frequency
(IF) signals. The mixing down converts a desired RF signal into a
desired IF signal having a desired IF center frequency. Generally,
it is easier to filter out unwanted signals that are close in
frequency to desired signals at IF frequencies than it is to filter
the same signals at higher RF frequencies. However, mixers have a
characteristic that produces image signals in addition to desired
signals. Image signals may be removed by RF filtering, IF
filtering, or both.
In any heterodyne receiver, when a received RF input signal F.sub.R
mixes with a local oscillator signal F.sub.LO, the mixer produces
an output signal with sums and differences of F.sub.R and F.sub.LO.
Specifically, the frequencies of F.sub.R-F.sub.LO and
F.sub.R+F.sub.LO, or F.sub.LO-F.sub.R and F.sub.R+F.sub.LO, are the
dominant mixer output frequency combinations. If F.sub.LO is chosen
with a lower frequency than a desired RF input signal F.sub.DRF,
then an F.sub.R-F.sub.LO portion of the mixer output signal
produces a desired IF signal F.sub.DIF; however, the mixer output
signal will also include an F.sub.R+F.sub.LO image signal, which is
close to double the frequency of F.sub.DRF and easily removed by IF
filtering. If a blocking image signal F.sub.BIS with a frequency
located at a frequency of F.sub.LO minus the frequency of F.sub.DIF
is received, the F.sub.R-F.sub.LO portion of the mixer output will
produce an image that is identical in frequency to F.sub.DIF but
phase-shifted 180 degrees from F.sub.DIF, and cannot be removed
with normal IF filtering techniques; therefore, if upstream RF
bandpass filtering cannot remove the blocking image signal, then
other techniques must be used to remove the signal. However, since
the blocking signal is phase-shifted by 180 degrees from F.sub.DIF,
a quadrature receiver architecture can be used to filter out the
blocking image signal. A quadrature receiver architecture uses two
mixers receiving the same RF input signal, which is mixed with two
different local oscillator signals that are equal in frequency and
phase-shifted from each other by 90 degrees. Complex filtering
methods can then be used to filter out the blocking image signal.
Any mismatch between the processing of in-phase signals and
quadrature-phase signals will result in degradation of the
rejection of image signals.
For example, if the desired RF input signal F.sub.DRF is at 900 Mhz
and the local oscillator signal F.sub.LO is at 899 Mhz, then the
desired IF signal F.sub.DIF is at 1 Mhz (F.sub.R-F.sub.LO).
Further, an IF image signal is at 1799 Mhz (F.sub.R+F.sub.LO),
which is easily filtered out in the IF section. A blocking image
signal F.sub.BIS at 898 Mhz will produce a blocking IF signal at -1
Mhz (F.sub.R-F.sub.LO), which is phase-shifted 180 degrees from
F.sub.DIF. If the blocking image signal F.sub.BIS cannot be
filtered out in the RF section, then complex filtering methods can
be used to filter out the blocking image signal in the IF
section.
Some RF communications protocols include as many channels as
possible in a given bandwidth; therefore, channel spacing may be
tight. As a result, desired IF center frequencies may be reduced to
maximize adjacent channel and alternate channel rejection. Some
communications systems use very low intermediate frequencies (VLIF)
or even down convert such that the desired IF center frequency is
zero, which is known as a direct conversion receiver (DCR);
however, lower IF center frequencies tend to produce certain side
effects. With a DCR, 1/f noise increases, direct current (DC)
offsets, and second-order inter-modulation (IIP2) effects may be
difficult to remove. As a result, effective receiver sensitivity
may be reduced. The optimum desired IF center frequency may vary
depending on the signal strengths of desired channels, adjacent
channels, and alternate channels.
In some networks, there is a loose correlation between the signal
strength of a desired signal and the signal strength of interfering
image signals; therefore, when the signal strength of a desired
signal is small, a higher desired VLIF frequency is desirable to
increase receiver sensitivity. The resulting reduced image
rejection is acceptable, since the signal strengths of interfering
image signals are also small. Likewise, when signal strengths of
interfering image signals are large, a lower desired VLIF frequency
is desirable to increase image rejection. The resulting reduced
receiver sensitivity is acceptable, since the signal strength of
the desired signal is also large; therefore, in some networks, it
would be beneficial to have an inverse correlation of the desired
VLIF center frequency with signal strength.
Given the above factors, there is a need for an RF receiver that
can change its desired IF center frequency based on signal
strengths of received signals. Additionally, there is a need for a
quadrature RF receiver with complex filtering having matched
circuitry processing the in-phase signals and the quadrature-phase
signals to eliminate blocking image signals that cannot be filtered
out in the RF section of the quadrature RF receiver.
SUMMARY OF THE INVENTION
The present invention is an RF receiver that uses an RF mixer for
tuning to desired frequency bands. The RF receiver down converts a
received RF signal into a VLIF signal. When receiving a desired RF
signal, the frequency of the resulting VLIF signal is called the
desired VLIF frequency, and is based on the signal strength of the
received RF signal. In one embodiment of the present invention, the
desired VLIF frequency is selected to be one of two VLIF
frequencies. The desired VLIF frequency is inversely related to the
signal strength of the received RF signal. For example, a higher
desired VLIF frequency is selected when receiving lower signal
strength RF signals to increase effective receiver sensitivity. The
higher VLIF frequency reduces de-sensitization due to 1/f noise, DC
offsets, inter-modulation effects, or any combination thereof. A
lower desired VLIF frequency is selected when receiving higher
signal strength RF signals to improve image rejection. The lower
VLIF frequency improves rejection of blocking image signals by
moving the VLIF frequency of the blocking image signal away from
the desired VLIF frequency, which allows IF filtering of some of
the blocking image signal.
Certain embodiments of the present invention may use a quadrature
RF mixer and quadrature polyphase filters to reject image
interfering signals. Certain embodiments of the present invention
may use quadrature gain correction circuitry, quadrature phase
correction circuitry, or both to match the circuitry processing the
in-phase signals and the quadrature-phase signals to improve image
rejection. Certain embodiments of the present invention may convert
the quadrature receiver signals into digital signals using
analog-to-digital (A-to-D) conversion. Digital circuitry may
provide polyphase filtering, down conversion, gain correction,
phase correction, processing, or any combination thereof.
Those skilled in the art will appreciate the scope of the present
invention and realize additional aspects thereof after reading the
following detailed description of the preferred embodiments in
association with the accompanying drawing figures.
BRIEF DESCRIPTION OF THE DRAWING FIGURES
The accompanying drawing figures incorporated in and forming a part
of this specification illustrate several aspects of the invention,
and together with the description serve to explain the principles
of the invention.
FIG. 1 shows one embodiment of the present invention used in a
quadrature RF receiver.
FIG. 2 shows details of the RF mixer circuitry of FIG. 1.
FIG. 3 shows details of the quadrature filtering and gain
correction circuitry of FIG. 1.
FIG. 4 shows details of the A-to-D conversion, digital filtering,
down conversion, and processing circuitry of FIG. 1.
FIG. 5 shows a graph of the frequency response of a 5-pole
polyphase bandpass filter having a VLIF center frequency at 175
kilohertz.
FIG. 6 shows an application example of the present invention used
in a mobile terminal.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The embodiments set forth below represent the necessary information
to enable those skilled in the art to practice the invention and
illustrate the best mode of practicing the invention. Upon reading
the following description in light of the accompanying drawing
figures, those skilled in the art will understand the concepts of
the invention and will recognize applications of these concepts not
particularly addressed herein. It should be understood that these
concepts and applications fall within the scope of the disclosure
and the accompanying claims.
The present invention is a radio frequency (RF) receiver that uses
an RF mixer for tuning to desired frequency bands. The RF receiver
down converts a received RF signal into a very low intermediate
frequency (VLIF) signal. When receiving a desired RF signal, the
frequency of the resulting VLIF signal is called the desired VLIF
frequency, and is based on the signal strength of the received RF
signal. In one embodiment of the present invention, the desired
VLIF frequency is selected to be one of two VLIF frequencies. The
desired VLIF frequency is inversely related to the signal strength
of the received RF signal. For example, a higher desired VLIF
frequency is selected when receiving lower signal strength RF
signals to increase effective receiver sensitivity. The higher VLIF
frequency reduces de-sensitization due to 1/f noise, DC offsets,
inter-modulation effects, or any combination thereof. A lower
desired VLIF frequency is selected when receiving higher signal
strength RF signals to improve image rejection. The lower VLIF
frequency improves rejection of blocking image signals by moving
the VLIF frequency of the blocking image signal away from the
desired VLIF frequency, which allows intermediate frequency (IF)
filtering of some of the blocking image signal.
Certain embodiments of the present invention may use a quadrature
RF mixer and quadrature polyphase filters to reject image
interfering signals. Certain embodiments of the present invention
may use quadrature gain correction circuitry, quadrature phase
correction circuitry, or both to match the circuitry processing the
in-phase signals and the quadrature-phase signals to improve image
rejection. Certain embodiments of the present invention may convert
the quadrature receiver signals into digital signals using
analog-to-digital (A-to-D) conversion. Digital circuitry may
provide polyphase filtering, down conversion, gain correction,
phase correction, processing, or any combination thereof.
FIG. 1 shows one embodiment of the present invention used in a
quadrature RF receiver 10. An RF input signal RF.sub.IN is received
by an RF amplifier 12, which buffers the RF input signal RF.sub.IN
to create a buffered RF input signal RF.sub.INB. RF mixer circuitry
14 receives and splits the buffered RF input signal RF.sub.INB into
two signals, which are mixed with quadrature local oscillator
signals to create a first in-phase down converted output signal
DC.sub.1I and a first quadrature-phase down converted output signal
DC.sub.1Q. The RF mixer circuitry 14 includes a frequency
synthesizer to create the quadrature local oscillator signals using
a reference frequency signal REFFREQ. The RF mixer circuitry 14
receives a frequency select signal FREQSEL to select the frequency
of the quadrature local oscillator signals.
The frequency of the quadrature local oscillator signals is
selected to be either higher or lower than the frequency of a
desired RF input signal RF.sub.IN such that the first down
converted output signals DC.sub.1I, DC.sub.1Q are VLIF signals with
a desired VLIF frequency. The desired VLIF frequency is inversely
related to the signal strength of the RF input signal RF.sub.IN,
and may be one of two VLIF frequencies; therefore, the frequency of
the quadrature local oscillator signals is selected to provide the
desired VLIF frequency. In an exemplary embodiment of the present
invention, when the signal strength of the RF input signal
RF.sub.IN is strong, the desired VLIF frequency is approximately
120 kilohertz (khz), and when the signal strength of the RF input
signal RF.sub.IN is weak, the desired VLIF frequency is
approximately 175 khz. Alternate embodiments of the present
invention may use any number of desired IF frequencies, which may
or may not include 120 khz, 175 khz, or both.
The RF mixer circuitry 14 feeds the first down converted output
signals DC.sub.1I, DC.sub.1Q into quadrature filtering and gain
correction circuitry 16, which filters out unwanted signals and
matches the in-phase signals and the quadrature-phase signals to
create a filtered in-phase down converted output signal DC.sub.FI
and a filtered quadrature-phase down converted output signal
DC.sub.FQ. The quadrature filtering and gain correction circuitry
16 feeds the filtered down converted output signals DC.sub.FI,
DC.sub.FQ into A-to-D conversion, digital filtering, down
conversion, and processing circuitry 18, which converts the
filtered down converted output signals DC.sub.FI, DC.sub.FQ from
analog signals into digital signals. The digital signals are
digitally filtered to remove adjacent channels, images, and any
other interfering signals. Any needed down conversion,
de-modulation, or signal processing is performed on the digital
signals. Signal strengths of desired and interfering signals may be
measured and provided in an RF signal strength signal RSSI. Any
required mode or control information is received from a digital
control signal DIGCONT. Control circuitry 20 receives the RF signal
strength signal RSSI, and then chooses the appropriate frequency of
the quadrature local oscillator signals based on the RF signal
strength signal RSSI. The control circuitry 20 provides the
frequency select signal FREQSEL and the digital control signal
DIGCONT with the proper information.
FIG. 2 shows details of the RF mixer circuitry 14 of FIG. 1. An
in-phase mixer 22 and a quadrature-phase mixer 24 receive the
buffered RF input signal RF.sub.INB. A synthesizer and local
oscillator 26 receives the reference frequency signal REFFREQ to
support synthesis of any needed local oscillator frequency, and the
frequency select signal FREQSEL to select the frequency of the
quadrature local oscillator signals. The synthesizer and local
oscillator 26 provides an in-phase local oscillator signal to the
in-phase mixer 22, and a quadrature-phase local oscillator signal
to the quadrature-phase mixer 24. The in-phase and quadrature-phase
local oscillator signals are approximately equal in amplitude and
phase-shifted approximately 90 degrees from each other. The mixers
22, 24 mix the local oscillator signals with the buffered RF input
signal RF.sub.INB to create the first down converted output signals
DC.sub.1I, DC.sub.1Q.
FIG. 3 shows details of the quadrature filtering and gain
correction circuitry 16 of FIG. 1. The first down converted output
signals DC.sub.1I, DC.sub.1Q feed quadrature gain and phase
correction circuitry 30, which applies gain and phase correction
factors to the first down converted output signals DC.sub.1I,
DC.sub.1Q to create a second in-phase down converted output signal
DC.sub.2I and a second quadrature-phase down converted output
signal DC.sub.2Q. The second down converted output signals
DC.sub.2I, DC.sub.2Q are approximately equal in amplitude and
phase-shifted 90 degrees from each other, which provides optimal
downstream complex filtering. The second down converted output
signals DC.sub.2I, DC.sub.2Q feed a quadrature polyphase VLIF
filter 32, which provides additional filtering of VLIF image
signals to create the filtered down converted output signals
DC.sub.FI, DC.sub.FQ. Alternate embodiments of the present
invention may include gain correction circuitry, phase correction
circuitry, or both, in the synthesizer and local oscillator 26, in
the mixers 22, 24, downstream of the mixers 22, 24, or any
combination thereof.
FIG. 4 shows details of the A-to-D conversion, digital filtering,
down conversion, and processing circuitry 18 of FIG. 1. The
filtered down converted output signals DC.sub.FI, DC.sub.FQ feed an
in-phase A-to-D converter 34 and a quadrature-phase A-to-D
converter 36, which convert the analog signals DC.sub.FI, DC.sub.FQ
into a digital in-phase down converted output signal DIG.sub.I and
a digital quadrature-phase down converted output signal DIG.sub.Q.
The A-to-D converters 34, 36 feed the digital down converted output
signals DIG.sub.I, DIG.sub.Q into digital filtering, down
conversion, and processing circuitry 38. The digital signals
DIG.sub.I, DIG.sub.Q are digitally filtered to remove adjacent
channels, images, and any other remaining interfering signals. Any
needed down conversion, de-modulation, or signal processing is
performed on the digital signals. Signal strengths of desired and
interfering signals may be measured and provided in the RF signal
strength signal RSSI. Any required mode or control information is
received from the digital control signal DIGCONT. Other embodiments
of the present invention may eliminate all or part of the
quadrature filtering and gain correction circuitry 16; however,
since all filtering and image rejection would need to be handled by
the digital filtering, down conversion, and processing circuitry
38, A-to-D converters 34, 36 with larger dynamic ranges may be
required.
In an exemplary embodiment of the present invention, the quadrature
RF receiver 10 uses one of two desired VLIF frequencies, which are
approximately 120 khz and 175 khz. An alternate channel blocker may
present an interfering RF signal at approximately 400 khz from a
desired RF signal. Other interfering systems may have an
interfering RF signal at approximately 600 khz from the desired RF
signal. The A-to-D converters 34, 36 may have a dynamic range of
approximately 85 decibels (db) to handle the dynamic range of
desired signals and remaining interfering signals. Without a
quadrature polyphase VLIF filter 32, the dynamic range of the
A-to-D converters 34, 36 would have to handle the full dynamic
ranges of interfering image signals, which may be approximately 95
db for the 400 khz interfering signal and 104 db for the 600 khz
interfering signal. Such wide dynamic ranges would increase the
cost, complexity, and current consumption of the A-to-D converters
34, 36; therefore, the quadrature polyphase VLIF filter 32 reduces
interfering image signals sufficiently to be handled by the A-to-D
converters 34, 36 with a dynamic range of approximately 85 db.
FIG. 5 shows a graph of the frequency response of a 5-pole
polyphase bandpass filter having a VLIF center frequency at 175
khz. 400 khz alternate channel blockers would produce images at 575
khz (175+400 khz) and -225 khz (175-400 khz). From FIG. 5, the
polyphase bandpass filter would attenuate these images by about 30
db; however, the frequency response curve illustrated in FIG. 5
assumes a perfect amplitude match and a perfect 90 degree
phase-shift between the in-phase side and the quadrature-phase side
of the polyphase bandpass filter. Any imbalances between the two
sides degrade the ability of the polyphase bandpass filter to
distinguish between -225 khz and +225 khz. Since +225 khz is
separated from 175 khz by only 50 khz, any +225 khz signals are in
the passband of the polyphase bandpass filter, and must be handled
by downstream circuitry; therefore, closely matched quadrature
circuitry may be required. Alternatively, a 5-pole polyphase
bandpass filter having a VLIF center frequency at 120 khz could be
used. 400 khz alternate channel blockers would produce images at
520 khz (120+400 khz) and -280 khz (120-400 khz). Quadrature
imbalances may produce a positive image at +280 khz, which is
separated from 120 khz by 160 khz and easier to filter out than a
signal with only 50 khz of separation. Therefore, use of a 120 khz
VLIF center frequency may reduce or eliminate the need for
additional quadrature balancing circuitry.
In an exemplary embodiment of the present invention, a 400 khz
alternate channel blocker may be 41 db stronger than a desired RF
signal when the signal strength of the desired RF signal is greater
than -83 decibel milliwatt (dbm). If the desired RF signal strength
is less than or equal to -83 dbm, then no alternate channel blocker
is specified. The -83 dbm threshold may be reduced another 10 db to
-93 dbm due to power control requirements. Use of a 175 khz VLIF
center frequency when the desired RF signal strength is less than
or equal to -93 dbm reduces receiver de-sensitization due to 1/f
noise, DC offsets, or inter-modulation effects; however, image
rejection is reduced, which is acceptable since no alternate
channel blockers are specified. The increased receiver sensitivity
is needed since the signal strength is lower. Switching to a 120
khz VLIF center frequency when the desired RF signal strength is
greater than -93 dbm improves image rejection, which may be needed
since alternate channel blockers may be present. The reduced
receiver sensitivity is acceptable since the signal strength is
higher.
Alternate embodiments of the present invention may use different
signal strength thresholds, or may use multiple desired VLIF
frequencies selected using multiple signal strength thresholds. One
embodiment of the present invention may use a continuously variable
desired VLIF frequency based on signal strength. Certain
embodiments of the present invention may select the frequency of
the quadrature local oscillator signals to be either higher or
lower than the frequency of the desired RF input signal RF.sub.IN
during the VLIF mode of operation. The selection may be based upon
which frequency reduces the magnitude of interfering signals, as
indicated by the signal strength of the RF input signal
RF.sub.IN.
An application example of a quadrature RF VLIF receiver is its use
in down conversion and digitization circuitry 40 in a mobile
terminal 42, the basic architecture of which is represented in FIG.
6. The mobile terminal 42 may include a receiver front end 44, a
radio frequency transmitter section 46, an antenna 48, a duplexer
or switch 50, a baseband processor 52, a control system 54, a
frequency synthesizer 56, and an interface 58. The receiver front
end 44 receives information bearing radio frequency signals from
one or more remote transmitters provided by a base station. A low
noise amplifier (LNA) 60 amplifies the signal. A filter circuit 62
minimizes broadband interference in the received signal, while the
down conversion and digitization circuitry 40 downconverts the
filtered, received signal to an intermediate or baseband frequency
signal, which is then digitized into one or more digital streams.
The baseband processor 52 provides mode and channel information to
the down conversion and digitization circuitry 40. The receiver
front end 44 typically uses one or more mixing frequencies
generated by the frequency synthesizer 56. The baseband processor
52 processes the digitized received signal to extract the
information or data bits conveyed in the received signal. This
processing typically comprises demodulation, decoding, and error
correction operations. As such, the baseband processor 52 is
generally implemented in one or more digital signal processors
(DSPs).
On the transmit side, the baseband processor 52 receives digitized
data, which may represent voice, data, or control information, from
the control system 54, which it encodes for transmission. The
encoded data is output to the transmitter 46, where it is used by a
modulator 64 to modulate a carrier signal that is at a desired
transmit frequency. A power amplifier system 66 amplifies the
modulated carrier signal to a level appropriate for transmission,
and delivers the amplified and modulated carrier signal to the
antenna 48 through the duplexer or switch 50.
A user may interact with the mobile terminal 42 via the interface
58, which may include interface circuitry 68 associated with a
microphone 70, a speaker 72, a keypad 74, and a display 76. The
interface circuitry 68 typically includes analog-to-digital
converters, digital-to-analog converters, amplifiers, and the like.
Additionally, it may include a voice encoder/decoder, in which case
it may communicate directly with the baseband processor 52. The
microphone 70 will typically convert audio input, such as the
user's voice, into an electrical signal, which is then digitized
and passed directly or indirectly to the baseband processor 52.
Audio information encoded in the received signal is recovered by
the baseband processor 52, and converted by the interface circuitry
68 into an analog signal suitable for driving the speaker 72. The
keypad 74 and display 76 enable the user to interact with the
mobile terminal 42, input numbers to be dialed, address book
information, or the like, as well as monitor call progress
information.
Those skilled in the art will recognize improvements and
modifications to the preferred embodiments of the present
invention. All such improvements and modifications are considered
within the scope of the concepts disclosed herein and the claims
that follow.
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